A lithium ion secondary battery has high energy density and is capable of being operated at a high potential, in comparison to secondary batteries such as a Ni—Cd battery and a Ni-MH battery. Therefore, the lithium ion secondary battery is widely used in small information equipment such as a mobile phone or a laptop. Since reducing the size and the weight of the lithium ion secondary battery is easily achieved, a demand as a secondary battery for hybrid vehicles and electric vehicles is increased. When being used for vehicles and the like, high safety is required. Thus, an all solid lithium ion secondary battery which does not use a flammable electrolyte is researched and developed. High ion conductivity is required for a solid electrolyte used in the all solid lithium ion secondary battery.
It is reported that a material having a cubic garnet-related structure has high ion conductivity (Patent Document 1). Thus, research and development of a material having such a structure are in progress. In particular, it is reported that a material of a chemical composition Li7-xLa3Zr2-xNbxO12 has high lithium ion conductivity in the vicinity of x=0.25, and a material of a chemical composition Li7-xLa3Zr2-xTaxO12 has high lithium ion conductivity in the vicinity of x=0.5.
In order to realize high lithium ion conductivity, it is necessary that grain boundary resistance or interfacial resistance is significantly reduced. Therefore, it is desirable that the solid electrolyte includes a solid material which is a high-density molded article. The solid material which is the high-density molded article is capable of preventing an occurrence of a short-circuit between a positive electrode and a negative electrode during charging and discharging, and the solid material can be thinned. Thus, reduction in size of the all solid lithium ion secondary battery may be allowed. However, it is known that the material having a cubic garnet-related structure has difficulty in being sintered, and it is difficult to produce a high-density molded article by using the material.